1. Field of the Invention
The present invention relates to a golf ball having an excellent flight performance and a good shot feeling, and more specifically, relates to an improvement in a resin composition to be used for an intermediate layer of a multi-layered golf ball.
2. Description of the Related Art
As a base resin used in an outer layer for covering a core of a golf ball, such as a cover layer or an intermediate layer, ionomer resin and polyurethane are used. A thread-wound core or a vulcanized rubber-molded core is wrapped with a resin composition containing these resins as a main component.
In the above structure, in order to achieve a long ball distance, a cover layer is preferably formed as an outer layer having high hardness and high stiffness to provide high resilience. However, such a cover layer having high hardness and high stiffness has a problem that it tends to get cracked after repeated hits and thus its durability deteriorates.
As a countermeasure, a multi-layered golf ball in which an outer layer surrounding a core is constituted with a cover layer and an intermediate layer has been proposed (Japanese Patent Publication No. 2910516).
Meanwhile, a ball distance of a golf ball depends on an initial speed, launch angle and a spin rate. However, a combination of a cover layer having high hardness and high stiffness with an intermediate layer having low hardness makes it difficult for powerless aged players or female players to increase the initial speed due to their swings in a slow speed and also to raise the launch angle of the golf ball. Thus, a sufficiently long ball distance is not achieved.
In addition, in order to decrease the spin rate in the multi-layered golf ball described above, it is preferable to provide the golf ball having a hardness gradient with larger hardness from a core toward an outer layer. Consequently, a resin to be used for an intermediate layer is softer than that for a cover layer, making it further unfavorable to improve the ball distance.
On the other hand, when both an intermediate layer and a cover layer have high hardness in order to improve a ball distance, such hard layers cause poor shot feeling and thus the above-mentioned players give low evaluation for such golf ball.
In an attempt to solve the problems described above, it has been proposed to include fibrous aluminum borate whiskers or the like in a cover layer (Japanese Laid-Open Patent Publication No. 10-137365). However, these fibrous substances have small specific surface areas. Because of such properties, although the fibrous substances are easily dispersed, the large-sized reinforcing materials come to scatter over the resin. Thus, when dispersed in the resin, the reinforcing materials have a small reinforcing effect. Therefore, in order to satisfy a desired mechanical property, it is required to blend a large amount of such materials. As a result, since the resin content in the resin composition is reduced, neither the high resilience being important as a golf ball nor the durability at a desired level is achieved.
Further, as an improvement for an intermediate layer, it has been proposed to include inorganic fillers such as barium sulfate and titanium white in order to enhance the resilience of the intermediate layer having low hardness (Japanese Laid-Open Patent Publication No. 2000-51396 and Japanese Laid-Open Patent Publication No. 2000-70411). However, since these fillers are not easily dispersed in the resin and the inorganic fillers raise the hardness, the shot feeling is lowered.
As a reinforcing material other than those described above, it has also been proposed to add nanocomposite material such as hydrotalcite or octosilicate as a filler into a resin composition (Japanese National Publication No. 2004-504900). Since the fillers of these types are formed into nano-sized fine particles, they are hard to be dispersed into resins. In particular, such fillers are difficult to be dispersed as particles into hydrophobic resins such as ionomer resin and polyurethane to be used for an intermediate layer. As a result, agglomerated particles are unevenly located in the resin, as is the case of the reinforcing materials having large particle diameters. Therefore, it has become apparent that, even if these reinforcing materials are used, it is impossible to obtain the properties of desired levels.